Refactor GlobalPrevalenceByTimeHandler as part of PrevalenceByLocationAndTimeHandler

[flaneuse]

GlobalPrevalenceByTimeHandler is the same handler as PrevalenceByLocationAndTimeHandler, but with location_id:None. So, we can get rid of it!

Steps to remove:

1. Allow PrevalenceByLocationAndTimeHandler to accept an empty location_id
2. Remove GlobalPrevalenceByTimeHandler
3. Remove the imports from https://github.com/outbreak-info/outbreak.api/blob/master/web/handlers/genomics/__init__.py
4. and genomics.py